Final answer:
Stimulating the tail of an Aplysia causes synaptic inhibition in the motor neuron of the gill, as inhibitory neurotransmitters lead to hyperpolarization of the presynaptic membrane and a decrease in action potential firing.
Step-by-step explanation:
Stimulating the tail of an Aplysia causes synaptic inhibition in the motor neuron of the gill. When the action potential reaches the axon terminals, neurotransmitter molecules are released into the synaptic cleft. If the neurotransmitters are inhibitory, like GABA (gamma-aminobutyric acid), they bind to receptors and cause a hyperpolarization of the presynaptic membrane through the opening of Cl- channels, making the neuron less likely to fire an action potential. This synaptic inhibition results from the neurotransmitter binding that leads to an inhibitory postsynaptic potential (IPSP). Conversely, excitatory neurotransmitters like acetylcholine at neuromuscular junctions lead to depolarization and muscle contraction. Therefore, the correct answer to this question is A. Inhibition.